NBSIR 73-347

ACTIVE AND PASSIVE MODE LOCKING OF

CONTINUOUSLY OPERATING RHODAMINE 6G DYE LASERS

Andre Scavennec

Norris S. Nahman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

February 1974

NBSIR 73-347

ACTIVE AND PASSIVE MODE LOCKING OF

CONTINUOUSLY OPERATING RHODAMINE 6G DYE LASERS

Andre Scavennec

Norris S. Nahman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

February 1974

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

NBSIR 73-347

ACTIVE AND PASSIVE MODE LOCKING OF

CONTINUOUSLY OPERATING RHODAMINE 6G DYE LASERS

Andre Scavennec

Norris S. Nahman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

February 1974

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

CONTENTS

Ch apter Page

1. INTRODUCTION 2

2. THE CW Rh 6G LASER -- 3

3. MODE LOCKING IN A LASER 11

4. ACTIVE MODE LOCKING OF THE CW Rh 6G LASER 15

5. PASSIVE MODE LOCKING OF THE CW Rh 6G LASER---- 22

6. COMPARISON OF THE TWO TECHNIQUES 2 8

7. CONCLUSION 30

REFERENCES-- 34

LIST OF FIGURES AND TABLES

Figure 2-1. Energy level diagram o£ Rh 6G 36

Figure 2-2. CW dye laser 37

Figure 2-3. Dye cells 38

Figure 2-4. Dye circulating system 39

Figure 3-1. Laser modes and mode locked output 40

Figure 4-1. Active mode locking, experimental setup-- 41

Figure 4-2. Acoustooptic modulator 42

Figure 4-3. Modulator driving circuit 43

Figure 4-4. Optical autocorrelation pulse width measurement setup 44

Figure 4-5. Active mode locking, detected pulse

waveform - 45

iii
LIST OF FIGURES AND TABLES (Continued)

Page Figure 4-6. Optical autocorrelation measurement

result 46

Figure 5-1. Passive mode locking, experimental setup- 47

Figure 5-2. Passive mode locking, detected pulse

waveforms 48

Table 6-1. Comparison of the results obtained with

the two mode locking techniques 49

iv
FOREWORD

This report summarizes the research performed by Dr.

Andre Scavennec during his stay at the National Bureau of

Standards, Boulder, Colorado, as a visiting guest scientist

from September 1972 to August 1973. Dr. Scavennec's home

laboratory is the Centre National D' Etudes des Telecommunications, Issy-les-Moulineaux, France. This report is typical

of the high quality research performed in the international

scientific exchange programs that NBS participates in and

strongly supports. The major discovery of this research,

namely the three mirror cavity for a passive mode -locked dye

laser, has been reported to the world-wide scientific community. A letter describing this discovery will be published

in the January 19 74 issue of the IEEE Journal of Quantum

Electronics

The second author. Dr. Norris S. Nahman, was the section

chief of the Pulse and Time Domain Section in which Dr.

Scavennec performed his research. Dr. Nahman provided

scientific counseling and direction of this research effort.

He is not longer with NBS. He is presently the chairman

of the Electrical Engineering Department of the University

of Toledo, Toledo, Ohio.

James R. Andrews

v

ACTIVE AND PASSIVE MODE LOCKING

OF CONTINUOUSLY OPERATING RHODAMINE

6G Dye Lasers

Andre Scavennecc* and N.S. Nahman**

ABSTRACT

Using confined and unconfined fluid flow dye

cells with suitable mode locking methods continuously

operating Rhodamine 6G dye lasers have been built

to produce narrow pulses (less than 40 ps) at about

a 140 MHz pulse repetition rate. For active (acoustic) amplitude modulation mode locking methods optical pulses were obtained having about a 32 ps pulse

width (FWHM) . For passive mode locking methods optical pulses of _< 35 ps pulse width (FWHM) were obtained

A three mirror cavity was used for the active

mode locking studies while three and five mirror

cavities were used in the passive studies. The three

mirror passive mode locked laser employed a single

dye cell containing both the Rhodamine 6G (gain) and

DODCI (loss) solutes in a single solution having

glycol as the solvent. The active and passive mode

o locked carrier wavelengths were about 5900 A and

o 5800 A, respectively.

Keywords: DODCI, dye laser; laser; mode-lock;

picosecond; Rhodamine 6G.

*Dr. Scavennec was an NBS guest scientist during 1972-73.

He is with the Centre National E' Etudes des Telecommunica

tions , Issy- les-Moulineaux, France.

*Dr. Nahman was formerly the section chief of the NBS Puis

and Time Domain Section. He is presently with the Electr

cal Engineering Department, University of Toledo, Toledo,

Ohio.
1. INTRODUCTION

The mode locking of the continuously operating (CW)

rhodamine 6G laser was recently reported with production

of extremely short optical pulses: about 50 ps duration

(F.W.H.M.) using active mode locking (acous tooptic amplitude

modulator) [1] , and a few ps ('^^ 2) using passive mode locking

(with Die thyloxadicarbocyanine Iodide or DODCI as the

saturable absorber) [2].*

One of the possible uses of so short optical pulses is

in the field of fast electronic metrology. Optical pulses

can be used either to drive a gate or to generate electronic

pulses. Optical pulses with width in the picosecond range,

with a repetition rate high enough to allow averaging or

sampling techniques,** would allow a substantial improvement in the field of the ultrafast electronic measurement.

The production of the picosecond pulses is of course

dependent on the achievement of very fast optoelectronic

transducers, for which one requires a fast response rather

than a good sensitivity, which is somehow the opposite of

what optical communications require.

*Other techniques than mode locking may be used to produce

short optical pulses : for instance 21 ps duration optical

pulses were obtained with a He-Ne laser for which a FabryPerot electrooptic modulator was set in place of one of the

cavity mirrors [3].

**Short ('^ 7 ps) and quite large (> 100 V) optical pulses

were obtained through optical rectification of powerful

pulses (.5 GW) emitted by a mode locked Nd doped glass

laser [4]. Unfortunately the repetition rate of such a laser is much too low to allow the use of sampling

techniques

.
This report only deals with the production of narrow

optical pulses; we report on the work done on the two techniques used to produce mode locking of the Rh 6G laser. The

obtained results are then compared with results from other

laboratories

2. THE CW Rh 6G LASER [5]

2 , 1 The Active Medium

The active medium is a solution of Rh 6G in a liquid

solvent

The population inversion is obtained through optical

pumping of the Rh 6G molecules and can be described by taking

into account four electronic vibrationally broadened levels,

two singlets and two triplets (fig. 2-1).

The molecules are excited from the lower level to the

excited level S^. They relax very rapidly to the thermal

distribution in and become available for amplification.

The relaxation of molecules to the thermal distribution in

is also a very fast process (10 sec). As a result, one

may consider the dye laser as a four level system though only

two levels (Sq and S.^^) are involved. The triplet states act

as trapping levels. Actually, the excited molecules in

can decay through spontaneous or stimulated emission down to

3
to Sq , or through non radiative process down to T^. The CW

operation of the Rh 6G laser was made possible only when a quencher was found for the triplet level [5] thus limiting

the lifetime of the molecules trapped in T^. The lifetimes

of the different transitions for a CW Rh 6G laser are

approximately [6] [7]:

Tg 5 . 5 ns

Tgrp 35 ns

Trj, 100 ns

The loss mechanisms limiting the gain of the laser are

the fluorescence loss (spontaneous emission) , the triplet

trapping, the T^ ^ T^ absorption and the self absorption

resulting from the overlap of the absorption and emission

bands. It is worth noting that the last process makes new

molecules available for the population inversion.

In order to reach the threshold for gain and lasing, it

o has been shown that, using the argon ion laser (5145 A) as

the pump, power density greater than 10 kW/cm^ is required.

This can be obtained by focusing the pump laser beam down to

a beam waist of approximately 10-30 ym in diameter. On the

other hand, losses are reduced by decreasing the length of the

active medium [8], As a result, the active medium is usually

confined in a small cylinder (20 ym in diameter, 1 mm or less

in length)

.

4
To prevent thermal distortion and dye degradation the

solution is flowed through the active region; the speed of the

flow is between 1 to 20 m/s leading to a residence time of 20

to 1 ysec for the dye molecules in the active region. In

order to minimize thermal distortion, a solvent with a low

thermal expansion coefficient is required.

2 . 2 The Laser Cavity

The dye laser and pump beams are usually collinear and

their widths are matched. This leads to a maximum gain for

the laser. However, for very thin active medium, the

collinearity is no longer required and a small angle is

allowed.

In order to have a cavity long enough to insert a mode

locking device and a beam waist about 20 ym diameter in the

active medium, either an internal lens or a third mirror is

required. The cavity we used, similar to the one described in

[9], is of the three mirror type (fig. 2-2).

In this cavity, the mirror curvature is chosen to focus

the pump beam to the desired size in the active medium. The

recollection mirror M-j^ curvature is chosen considering the

beam waist and ease of mechanical setup considerations. The

mirror is a long radius of curvature -- usually flat --

mirror. The large breadth of the emission band characteristic

5
of the dye lasers allows the broad frequency tuning of the

Rh 6G laser. The tuning is done through a dispersive element,

the Brewster angle prism P. The prism also allows the pump

beam to be injected into the cavity without going through

one of the cavity mirrors.

The angle 2 0 between the two arms of the cavity is determined to minimize the astigmatism of the cavity [10]; the

astigmatism resulting from the Brewster angle dye cell is

compensated by the astigmatism from the mirror M2 . For

usual solvents and cell thicknesses and for a curvature of

equal to 10 cm, 26 is of the order of 10°-15°.

2.3 The Dye Cell

The dye is conventionally flowed through a cell, two

windows with the desired spacing confining the dye solution,

while allowing the pump and the dye laser beams , crossing

the cell at the Brewster angle, to interact with the active

medium.

Recent development in dye laser technology showed that,

by the choice of a viscous solvent, it was possible to use

an unconfined film of dye solution as the active medium [1].

The solution is flowed through a nozzle designed so as to

give a liquid film of good optical quality.

In the course of our work, we used different cells of

the two kinds, i.e., the confined and unconfined.

6
2.3.1 The Confined Type Cell

This cell is similar to the one designed by D.A. Jennings

in the NBS Quantum Electronics Division (fig. 2-3) . The

only difference was in the choice of inexpensive microscope

cover glasses as windows instead of polished quartz. Actually the beam sizes are so small, that even conventional

microscope cover glass has sufficient flatness and optical

quality to be used inside the laser cavity [12]. However

it is worth noting that the poor thermal conduction of glass

often results in thermal problems and instabilities [5]

With that type of cell, we used a 4 x 10 '''' molar solution

of Rh 6G in water with 41 Ammonyx LO. The flow speed in

the active region was approximately 3 m/s

2.3.2 The Unconfined Type Cell

The instabilities we encountered resulting from the

poor thermal conduction of glass led us to develop cells of

unconfined type which are free of such defects.

Moreover using an unconfined type cell avoids window

degradation (deposition of burnt particules by the pump

beam) which is a persistent problem of confined type cells.

Most of the unconfined type cells we designed used a nozzle made of a copper tubing on which two razor blades were

stuck (fig. 2-3), the slit dimensions were approximately

6 X .5 mm. The liquid flowed through that nozzle must have

7
a high enough viscosity (> 10 cp) . We used ethylene glycol

(viscosity 20 cp at 20°C) and molar concentration o£ approximately 5 X 10 with a flow speed of 2 m/s . The free flowing

glycol film was estimated to be .2 mm thick in the active

region

.

Other attempts were made to design a pinched copper

tubing as the nozzle [11]. However the poor mechanical

quality of the edges resulted in a poor optical quality of

the film.

2.3.3 The Circulating Loop

The dye solution is flowed through the cell by means

of a magnetically driven pump. A filter is set in line to

remove small particules and bubbles.

For the confined type cell using water as the solvent,

the filter has pore size of 1.2 ym. With unconfined type

cells using high viscosity glycol, it is necessary to have

filters with larger pore sizes 7 ym. Since burnt particules

are no longer a problem, even larger pore filters might be

used (s ay 15 ym) .

The film produced by unconfined type cells is very sensitive to pump and other mechanical instabilities. We found

that damping the flow pulsations and the pump vibrations was

8
necessary for an acceptable stability o£ the film. This was

achieved through the use of corrugated tubing for vibration

isolation and of a damping box inserted between the pump and

the filter for the flow pulsations (fig. 2-4).

It was also necessary to use a cooling reservoir to

remove the laser heat from the glycol. This was necessary

to retain the required high viscosity.

2 . 4 Characteristics of the Laser

Most of the laser characteristics are related to the

confined cell laser.

o 1. The laser emission bandwidth goes from 5650 A up

o o to 6200 A for .5 W of pump power at 5145 A. No

saturation of the output/input power characteristic

could be seen up to 1.5 W of pump power, which means

that the flow speed is high enough.*

2. The cavity losses (diffraction, scattering ...)

were checked and found to be quite small: using

three maximum reflectance mirrors, the lasing

o threshold was only 30 mW (5145 A pump power)

.

The threshold with the free flowing film laser

was a little bit higher ["^ 50 mW) .

*This measurement was performed with three maximum reflectance

mirrors and the output was the reflected power from one of

the prism faces. In usual operation, a 4-5% transmission

mirror was used as the output mirror (M^^) .

9
3. At high pump power (> 1 W) a better stability was

achieved with the free flowing film laser. However,

the use of quartz windows and possibly cooling of

^ the solution [5] would improve greatly the stability

of the laser with the confined cell.

4. The laser output stability is a function of the cell

and flow stability. It is also dependent on the

pump laser stability and on a good optical isolation between the pump laser and the dye laser,

in order to prevent feedback and etalon effects.

5. The pump laser noise was < 0.5% peak to peak

(10 Hz - 2 MHz) and good isolation was provided by

the high absorption of the dye solution. The concentration was adjusted to give a 90-95% absorption

per pass , thus limiting the power reflected toward

the Ar laser to less than 1%.

6. In the best case, the noise of the dye laser output

was observed with a fast detector and a 100 MHz

bandwidth to reduce to 8% peak to peak. When the

laser is close to the threshold, the output power

is more sensitive to the pump laser noise and

becomes noisier.

10
3. MODE LOCKING IN A LASER

We now give a few characteristics of mode locking in a

laser. This had been the subject of extensive research, and

analysis in both time and frequency domain were published [13].

3 . 1 General Considerations

Only the laser modes associated with TEM modes of the ' ooq

laser cavity are considered. In the frequency domain, these

modes are almost equally spaced. The intermode frequency is

c/2L, the fundamental frequency of the laser cavity [y 100 MHz).

The only modes able to oscillate are the ones for which the

laser gain (the gain linewidth is usually 100 MHz < Av^^ < 100 GHz)

is greater than the losses (fig. 2-1).

The modes are oscillating with random phase and amplitudes with a weak intermode coupling. The output signal,

characterized by its electric field can be written (fig.

3-ld) :

E(t) = Z E^(t) e ^

^

n

th E^(t): amplitude of the n mode

t h

^n*^n* fi^equency and phase of the n mode

The intermode coupling resulting from the active medium nonlinearities can induce self mode locking (as for the He-Ne laser

o at 6328 A). In such a case, the modes are oscillating in

11
phase and with a steady, approximately gauss ian amplitude

distribution.

(jo = 0) 1 + f2 n n-1

^n ^n-1

-kn^ E (t) = E Ct) • e

^o^^-^

have small variations

The output signal is then

with time

E(t) = E^ e ° ^ e

Zn'^ + jnJ^t

n=-p

To this signal is associated an optical pulse train with

2L 1

T = ~ as the repetition period and t as the pulseL

width (fig , 3- le) . With most lasers, it is possible to produce a similar

mode synchronization, by adding nonlinearities inside the

laser cavity. One can use either an active modulator varying

the phases or amplitudes of modes at a frequency equal to the

intermode frequency or a "passive" element such as a saturable

absorber,

3 . 2 Frequency Dispersion of Modes

If the intermode frequencies were equal all over the

oscillation width (as are the empty cavity intermode frequencies)

the nonlinearities of the laser active medium would be sufficient by themselves to induce mode locking. Several dispersion sources actually prevent the modes from being equally

spaced:

12
1. The nonlinear variation of the active medium refractive index around its resonance results in dispersion

of the laser modes resonance frequencies (referring

to a linear distribution, the dispersion may be of

the order of a few kHz or MHz) . This dispersion

is quite large for inhomogeneous ly broadened laser

lines (gas lasers) ; however it is negligible for

homogeneously broadened laser lines such as dye

lasers [14]

2. Internal optical elements often act as etalons . The

well known property of etalons is to modulate the

amplitude of the cavity loss as a function of the

wavelength. However one must keep in mind that these

etalons are also frequency dispersive and can prevent mode locking [15]. It is then necessary to

take extra care to get rid of such etalons , either

by tilting optical elements or by good quality

anti-reflection coatings or, more efficiently, by

setting elements at the Brewster angle.

3. 3 Rh 6G Laser Mode Locking

The active medium nonlinearities are not large enough to

induce self mode locking. However the observation of the

dispersion of modes for the laser we built shows that relatively small nonlinearities are required to induce mode locking.

13
The RF spectrum analyzer observation of beat notes widths,

somewhat related to the dispersion [16], gives values around

60-100 kHz. Such values are quite similar to what is observed

o for the He-Ne 6328 A laser (40-100 kHz), and much smaller

than for the Ar Ion laser (300 kHz - 1.5 MHz).

3 . 4 Note on the Homogeneous Nature of the Rh 6G Active Medium

The homogeneous line of the Rh 6G dye laser is usually

o o considered to have a width of several A ('v 5 A) . This width

is larger than the oscillation width of the laser, resulting

from the dispersive nature of the cavity (prism) , As a result , the Rh 6G laser has essentially a homogeneously broadened

active medium. However due to the inhomogeneous spatial

saturation of the active medium by modes at different frequencies, there is some inhomogeneity : two modes, spatially

offset by 7t/2 in the dye cell will saturate the active medium

almost independently. That is why a monomode Rh 6G laser has

a tendency to oscillate on two modes, spaced c/4d apart, where

d is the distance between the cell and the mirror M^ [17].

The RF spectrum analyzer observation of the output of our laser

also shows intense beat notes between 1.2 and 1.7 GHz (c/4d =

1.5 GHz). The modulation frequencies we used in this study

were smaller than c/4d, and no problems were expected coming

from the inhomogeneous spatial saturation. However with modulation frequencies in the vicinity of c/4d one may expect to

observe parasitic phenomena such as simultaneous oscillation

of two independent pulse trains.

14
4. ACTIVE MODE LOCKING OF THE CW Rh 6G LASER

The laser cavity was similar to the one described in

the first part. However in order to have more room to set

the modulator, a second prism was added to separate the pump

and the dye laser beams (fig. 4-1) . One of the prisms was

a Brewster angle prism, the second was a 60° prism which

provided an additional output for triggering purposes. The

prisms were set so as to reduce the dispersion of the cavity.

4.1 The Modulator

The results reported in the literature on CW Rh 6G

laser active mode locking show that amplitude modulation [1]

leads to better results than phase modulation [18]. This

is to be expected from the fact that phase modulation is very

poor to prevent the oscillations of several intermingled

pulse trains corresponding to different etalons transmission

peaks. Moreover, for a same modulation index, amplitude

modulation gives narrower pulses than phase modulation [19].

As a result, we used an acoustooptic amplitude modulator.*

The modulator is made of a silica block (2 mm thick) in

which standing acoustic waves are generated by a rectangular

X-cut quartz crystal (23.4 MHz fundamental frequency, driven

at 70.36 MHz) (fig. 4-2). The acoustic standing waves

*0n loan from C.N.E.T.

15
modulate the refractive index. The optical beam propagating

into the silica interacts with the phase grating and sustains

losses at twice the ultrasonic frequency [20]. The optical

beam transmitted through such a modulator is:

I(t) ^^00 = I (v sin a)*t)

I(t) , I \ J f o J

o

0) *

V

I - <I(t)>

o ^

^

I o

Transmitted and incident beams

Bessel function

Ultrasonic frequency

Index of modulation

The modulator we used was built a few years ago for use in an

Ar Ion laser mode locking experiment, and slow degradation of

the bonding has reduced its efficiency down to approximately

for 40 mW of absorbed electrical power. This

efficiency was still sufficient for the Rh 6G laser mode

locking.

The modulator is driven by a very stable phase-locked

frequency source: in order to reduce the frequency drift resulting from the wide variations of the modulator impedance

around resonance. A good electrical isolation is provided between the oscillator and the modulator. Since the modulator

is very temperature sensitive it is necessary to monitor the

VSWR as a check on the modulator tuning (fig. 4-3).

16
4.2 The Detection Apparatus

1. Electronic Detection: The output signal was detected with a fast Si photodiode* (transition time

10%-90l 61 ps) . A slower (transition time

120 ps) commercial avalanche photodiode is used

for triggering. The avalanche photodiode was used

in conjuction with a RF spectrum analyzer to align

the cavity fundamental frequency and the modulation

frequency, and also for the first settings (modulator adjustment for maximum efficiency).

When good mode locking was evidenced by the RF

spectrum, the fast Si photodiode output was directed

onto a 28 ps sampler and a display oscilloscope.

Careful adjustments of the trigger signal level and

trigger stability were necessary to synchronize the

oscilloscope to the pulses.

2. Optical Detection: Since the width of the optical

pulses is approximately equal to the photodiode

transition time, a large uncertainty on the actual

pulse width results from the electronic measurement.

In order to have an accurate measurement of the

optical pulse width, an optical autocorrelation

measurement, similar to the one reported in [1] was

performed, using Second Harmonic Generation (SHG)

*0n loan from C.N.E.T.

17
in KDP (fig. 4-4). Such a measurement is similar

to the well known Two Photon Fluorescence (TPF)

measurement performed to measure pulses from flashlamp pumped lasers. The measurements were done

c O around 5900 A, which is the wavelength for which

the SHG crystal was cut, (no pulse width dependence

with frequency is expected) . Considerable care

was given to the alignment of the reflecting prisms

in order to avoid false results which might result

from non- collinear recombined beams,

4 . 3 Results of Active Mode Locking Experiments

Most of the experiments with active mode locking were

conducted with the free flowing film type of laser and with a

41 transmission mirror as M^ . Insertion of the modulator

inside the cavity raised the threshold up to 500 mW.* In

usual operation, the pump power was set to 1.0-1.2 W depending

on the nozzle and solution used. With the modulator operating,

the output goes down to 10-12 mW average power approximately,

from 50 mW without modulation. The use of a smaller modulator

very close to M^ would probably improve that figure.

*One may note that increasing the losses of the cavity increases the population in the excited singlet state and

thus increases the active medium loss due to the triplet

states

.

18
Single transverse mode operation was obtained by introduction of a small ('^' 2 mm diameter) iris into the cavity.

Good mode locking resulted from careful alignment of the

cavity and modulation frequencies, and a good stability of the

output pulses was obtained through dye cell adjustments. The

different waveforms we observed as we detuned the cavity

frequency were quite similar to what is observed with an

Ar Ion laser [16]. The short fluorescence lifetime characteristic of these two lasers might be the reason of those

observations [21], Only for the optimum frequency and for

a high enough modulation index did we observe pulsing with

a single pulse per modulation period; detuning in either direction resulted in multiple pulsing, usually with broader

pulses

With a 28 ps transition time sampler, the pulses we

observed with the sampling oscilloscope had a width around

70 ps (F.W.H.M.), and a risetime (10%-90l) of 50 ps , which

are consistent with a gaussian pulse (fig. 4-5).

The optically detected pulse widths (F.W.H.M.) were

measured to be 32± 2 ps , assuming a gaussian optical pulse.

On the figure 4-6 is a recording of the signal detected

by the photomultiplier (pulse autocorrelation function)

Because the translation length for the recording system was

limited to 10 mm, we scanned only part of the curve. However

19
we first checked by manual translation for symmetry of the

- curve. The contrast ratios don't exactly agree with theoretical predictions (3:2:1 for the central spike, the broader

spike, and the flat background): this is attributed to

slightly different beam sizes coming from the two interferometer arms. The sharp central spike is related to the coherence

of the laser (or the width of the oscillating spectrum) the

broader spike is related to the actual pulse width and the

background to the sum of the SHG signals resulting from each

interferometer arm signal.

From the two measurements, we were able to determine a transition time (101-90^) of 61 ps for the fast photodiode.

Larger pulse widths were reported by Dienes et al . [1],

with not so large a pulse width dependence on the modulation

frequency as predicted for homogeneously broadened lasers [19].

Their measured values would give, for the modulation frequency

we used (_'^ 140 MHz), a pulse width of 62 ps . It is not clear

to us why we obtained shorter pulses than theirs, though the

expected value of the mode locked Rh 6G laser pulse width

according to the theory developed in [19] is still much

shorter (a few picoseconds)

.

The optical measurement of the pulse widths was performed

when the laser was operating with a poor optical quality dye

film and other experiments with better dye films (thinner ?)

might give even better results.

20
However, one should note that the theory developed in [19]

applies to the case o£ an homogeneously broadened active

medium with a fluorescence lifetime much larger than 2L/^,

which is not at all the situation for the Rh 6G laser. Nonlinear amplification, not included in theory [19], is thought

to be of major importance for the dye laser and is probably

responsible for the larger pulse width.

21
5. PASSIVE MODE LOCKING OF THE CW Rh 6G LASER

A few years ago, passive mode locking of the flashlamp

pumped Rh 6G dye laser [22] gave pulses with a few picoseconds widths. The same absorber, diethyloxadicarbocyanine

Iodide CDODCI)was used to produce mode locking of the CW

laser [2] [23]

.

In order to drive the absorber into its nonlinear region,

focusing of the CW Rh 6G laser beam in the absorber is required to obtain a high enough power density. The cavity

of a mode locked CW Rh 6G laser has then to be designed

so as to produce small beam diameters in both the amplifying

and the absorbing medium.

This was done by Ippen, et al. [2] through the use of

a five mirror cavity which is an extension of the three

mirror cavity [9]. O'Neill [23] used an inline type of

cavity with lenses to focus the beam. The five mirror

cavity seems to give better results , so we decided to try

that type of configuration.

5 . 1 The Five Mirror Cavity Passively Mode Locked Laser

The confined type cell was used for the amplifying

medium (Rh 6G in water + Ammonyx LO) and the absorber (DODCI

in glycol) was flowed through a free flowing type cell. The

mirrors had the same curvature and reflectivity as the ones

22
used in [2], leading to beam diameter o£ 25 ym in the Rh 6G

solution and 15 ym in the DODCI solution. The cavity length

(1.60 m) was a bit shorter than in [2] (fig. 5-la)

.

We first had the three mirror cavity Rh 6G laser lasing.

The mirror was the tilted so as to direct the flurescence

pattern onto the mirror , and the five mirror cavity was

adjusted for lasing with the glycol film in the beam. DODCI

was then carefully added to the glycol. Most of the time,

extinction of the lasing action resulted and it was necessary

to readjust the laser cavity to obtain lasing again. This

probably results from the increase in absorption of the glycol

film and associated change in thermal focusing. It is our

belief though that a slow change in DODCI absorption may occur

after mixing with glycol (conversion to an isomeric configuration) which would explain that we always had to wait a little

bit after adding DODCI before the laser would oscillate again.

Degradation of DODCI (bleaching) actually occurred and approximately twice a week, it was necessary to add some DODCI to

the absorber solution.

Adjustment of the cavity parameters and of the absorber

concentration resulted in mode locking. The photodiode detected pulse widths were 72 ps (F.W.H.M.), as for the pulses

produced by active mode locking. However, due to large instabilities of the cells, the observed signal was quite noisy.

23
Only with smoothing and persistence on the display oscilloscope were we able to observe the detected pulse.* The average

output power had to be restricted to a few mW for clean

pulsing. Increasing the laser power --by increasing the

pump power -- resulted in multiple pulsing (several pulses

superimposed to a DC level per period) . Detuning of the

absorber cell from its best position led to an increase of

the pulse width.

The increase in pulse width seemed to be gradual with

detuning and we did not observe any sudden jump to a regime

with 300 ps wide pulses as reported in [2]. During this

experiment we had not the opportunity to carry on an optical

pulse width measurement.

In order to reduce the noise due to thermal instabilities

of the confined cell, we decided to try using the free flowing

type cell for the amplifying medium (Rh 6G in glycol) and the

confined cell for the absorber (DODCI in methanol). The confined cell, no longer exposed to the powerful pump laser, was

supposed to be more stable. However we were unable to obtain

good mode locking with that arrangement. This may have resulted from the wavelength shifts of Rh 6G and DODCI resonances

as a function of the solvent.

*This was the first experiment we conducted and the stability

has since then improved. The free flowing type cell was redesigned and the thermal instabilities of the confined cell

reduced by increasing the flow speed. Moreover a better

optical isolation was provided for the Ar Ion laser, thus

reducing its noise.

24
5 . 2 The Three Mirror Cavity Passively Mode Locked Laser

The five mirror arrangement described in the preceding

section is very sensitive to mechanical noise and adjustment

of the numerous settings is not easy.

In order to simplify alignment of the cavity we decided

to fold that five mirror arrangement and to have a single cell

through which both the amplifying and the absorbing media are

flowed (fig. 5-16).

Both Rh 6G and DODCI may be used with glycol as the solvent, so we chose a free flowing type of cell. The expected

problems were : bleaching and saturation of the DODCI absorption by

the Ar Ion pump laser;

reaction between Rh 6G and DODCI modifying the

properties of either one.

Actually no noticeable reduction of the mode locking

properties of DODCI resulting from saturation of the absorpo tion by the 5145 A pumping radiation could be seen, nor did

we observe a faster bleaching of DODCI than when used in a separate cell with the five mirror cavity configuration.*

The properties of either Rh 6G or DODCI didn't seem to be

impaired by the presence of each other in the same solution.

*It seems however that DODCI is bleaching faster in glycol than

in methanol. According A. Dienes , it is necessary to change

the methanol solution of DODCI only once a month in the

arrangement of [2], In our case, it's necessary to add some

DODCI to the solution approximately twice a week.

25
Good mode locking was obtained with the new arrangement.

The alignment o£ the cavity was as easy as for the simple

three mirror laser and this resulted in a much easier mode

locked source to work with.

We used molar concentrations o£ approximately 6 x lO''^

of Rh 6G and 5 x 10 ^ of DODCI , though no attempt has been

made to determine the best concentrations. Moreover, due

to the bleaching of DODCI, we added some from time to time,

so the absorber concentration is not really meaningful.

The observed mode locked pulses, detection limited, had

a width of 72 ps (F.W.H.M.) for the best settings (fig. 5-2).

Detuning the cell resulted in broader, more stable pulses;

the decrease of the stability with decreasing pulse width is

attributed to the broader oscillating bandwidth which makes

slight mechanical or pumping variations more critical. The

stability was much better than with the five mirror arrangement. However we still needed smoothing and persistence on

the display section. We later found that part of the problem

can be attributed to a poor triggering of the oscilloscope.

From the photodiode and sampler response times, we can infer

an upper limit to the optical pulse width to be around 35 ps

.

We didn't perform any optical measurement of the pulse width,

which we believe to be quite small (< 10 ps)

.

26
The average output power was 6-10 mW and mode locking

o o was obtained around 5 800 A with a 100 A tuning range. In

order to avoid multiple pulsing, the cavity length was chosen

so as to have a round trip time ("^ 6 ns) of the order o£

the recovery time of the laser gain. For the same average

power, an increase in the cavity length led to the presence

of a second pulse or more in the cavity; it was then necessary to reduce the pump power to obtain single pulsing again.

Tuning the laser from the orange toward the yellow resulted in strong CW oscillation, and we did not observe any

bistability as observed by Ippen, et al . [2]. However, when

the laser was tuned to the red, CW oscillation accompanied

by such a bistability was seen. When the lasing action was

interrupted, it was possible to initiate lasing again either

by tuning back to the orange and then continuously to the

o red, or by increasing the 5145 A pump intensity. Such a behavior is rather similar to the hysteresis observed by

Lisitsyn and Chebotaev [24] with a He-Ne laser and an intracavity Ne absorbing cell. Additional information on saturation parameters of Rh 6G and DODCI should be helpful in

interpreting the bistability phenomena.

We finally found that this three mirror arrangement gave

better results (mainly in stability and reproducibility) than

the five mirror arrangement tried before and further work on

it is expected to result in a very attractive source for the

continuous production of picosecond optical pulses.

27
6 . COMPARISON OF THE TWO TECHNIQUES

In this last section we plan to compare the results we

obtained with the two mode locking techniques.

A simple way to describe the results is to say that

o active mode locking leads to a widely tunable ('\> 800 A or

more) source of short optical pulses while passive mode

locking gives ultrashort optical pulses on a smaller bando width ('^ 100 A). In table 6-1 are summarized the values o£

the pulse widths, average output power, and peak power o£

the pulses. Also shown are the results published by other

laboratories.

For active mode locking it is worth noting the small

width (32 ps F.W.H.M.) we obtained as compared to the results o£ reference [1]. Increasing the efficiency of the

modulator and decreasing its size would allow operation at

higher average power. Since operating at higher frequency

leads to shorter pulse widths, we believe that 25 ps F.W.H.M.

optical pulses with peak power greater than 10 W may easily

be obtained on a broad bandwidth with our laser. Moreover

we don't know of any reason why the pulse width could not

be reduced to 10 ps or less, and more work in this direction

could lead to a large improvement in the pulse widths obtained

with active mode locking. However it is worth noting that

in active mode locking, the achievement of very narrow pulses

28
and the stability of the pulse train depends on the good

matching o£ the modulation frequency and cavity frequency.

The shorter the pulse width, the more critical is this

matching. A mismatch of 1 kHz (or cavity length variation

of 10 ym) actually results with our laser in a broadening

of the pulse. This effect would be much more severe with

10 ps pulses.

For that reason, one may prefer the passive mode locking

technique for which no frequency matching is required. The

other advantage of this second technique is in the ultrashort pulse widths observed in [2] and [23]. In our case

we had not the opportunity to make an accurate measurement of

the pulse width, but there is no reason why the pulses should

be larger than 10 ps . In the table 6-1, one can see that

the pulse peak power obtained with passive mode locking is

much higher than with active mode locking, which is another

reason to prefer the former technique. The latter however

is more likely to allow higher energy per pulse since passive

mode locking average output power seems to be limited to

10 W. The bleaching of DODCI also requires changing the

absorber solution from time to time, which is not really too

much of a problem.

29
7. CONCLUSION

The mode locking of the CW Rh 6G dye laser has been

presented. We have compared the two usual mode locking techniques, active mode locking (with an acoustooptic modulator)

and passive mode locking (with DODCI)

.

We would like to outline two points which give some

originality to our work in regard to previous work:

The observed pulse width using active mode locking

(35 ps F.W.H.M.) shows a real improvement of the

pulse width measured by Dienes , et al . [1], though

the modulation indices were almost the same. This

result shows that it is probably still possible to

improve the pulse width with active mode locking.

The realization of the laser using a single active

medium, solution of Rh 6G and DODCI in glycol,

allows the simultaneous production of gain and nonlinear absorption. Such a laser, corresponding to

a substantial simplification of the laser developed

by Ippen, et al. [2], has the great advantage to be

very easy to align.*

Some more work should lead to an improvement of the mode

locked lasers characteristics :

*A letter on that point is to be published in the Corresponden

section of the IEEE Journal of Quantum Electronics, January

1974.

30
The design of more stable cells should suppress

part of the laser noise.

A Perot- Fabry optical spectrum analyzer would be

helpful to assist in the search for shorter pulses.

A smaller modulator (with a deposited mirror on the

back surface) working at a higher frequency would

increase the capacities (shorter pulses , higher

peak power) of the active mode locked laser.

The mode locked Rh 6G laser used in conjunction with

ultrafast optoelectronic transducers should make possible

the achievement of recurrent electric pulse generations and

gates with faster response than presently available devices.

The study of photodetectors is presently carried on at

NBS. Among the different types are:

Photodiode detectors: The classical vacuum or

semiconductor photodiodes are limited by their

transit time and junction capacitance. Metallic

point contact photodiodes have a better potential

for ultrafast detection. These photodiodes are

now working in the visible [25]. The main defect

seems to be their short lifetime.

Optical rectification: The phenomena is very fast.

However high optical power densities are required,

and optical rectification probably can't be used in

conjunction with a CW Rh 6G mode locked laser.

31
Photoconductivity: Photoconductors have not the

transit time and junction capacitance limitations o£

classical photodiodes. The only limitation is with

the carrier lifetime, which may be quite small (a

few ps for GaAs) . Such photoconductors have already been realized [26], However they are not

very sensitive. An improvement of the sensitivity

seems necessary (for instance through the addition

of Al in order to move the absorption toward the

red) .

32
ACKNOWLEDGMENT

This work benefited from many fruitful discussions on

pulse techniques with Dr. J.R. Andrews, the help of Dr.

R.A. Lawton for optical pulse width measurements, and also

the help received from the staff of the Quantum Electronics

Division for the CW Dye Laser design.

One of the authors (A. Scavennec) appreciated the opportunity given to him to carry on this research project as

a Guest Worker with the Electromagnetics Division.

33
REFERENCES

[I] A.Dienes, E.P. Ippen, C.V. Shank, "A mode locked cw dye

laser," Appl. Phys . Letters, 19_, 258-260 , 1971.

[2] E.P. Ippen, C.V. Shank, A. Dienes , "Passive mode locking

of the cw dye laser," Appl. Phys. Letter, 2_1 , 348-350 , 19 72 .

[3] T. Kobayashi, T. Sueta, Y. Cho , Y. Matsuo , "Highrepetition- rate optical pulse generator using a FabryPerot electrooptic modulator," Appl. Phys. Letters, 2

1

, 341-343, 1972.

[4] D. H. Auston, A.M. Glass, "Optical generation of intense

picosecond electrical pulses," Appl. Phys. Letters, 20 , 398-399, 1972.

[5] O.G. Peterson, S.A. Tuccio, B.B. Snavely, "CW operation

of an organic dye solution laser," Appl. Phys. Letters,

17, 245-247, 1970.

[6] B.B. Snavely, "Flashlamp-excited organic dye laser,"

Proc. IEEE, 57_, 1374-1390 , 1969.

[7] M.E. Mack, "Measurement of nanosecond fluorescence

decay times," J. Appl. Phys., 39^, 2483-2485 , 1968.

[8] F.C. Strome , S.A. Tuccio, "Loss analysis and design

improvement for a continuous dye laser," IEEE J. Quantum

Electron., 9^, 230-235 , 1973.

[9] A. Dienes, E.P. Ippen, C.V. Shank, "High efficiency

tunable cw dye laser," IEEE J. Quantum Electron., 8^, 388, 19 72 .

[10] H.W. Kogelnik, E.P. Ippen, A. Dienes, C.V. Shank,

"Astigmatical ly compensated cavities for cw dye lasers,"

IEEE J. Quantum Electron., 8^, 373-379 , 1972.

[II] P.K. Runge , R. Rosenberg, "Unconfined flowing-dye films

for cw dye lasers," IEEE J. Quantum Electron., 8_, 910-

911, 19 72.

[12] A. Dienes, Private communication.

[13] P.W. Smith, "Mode locking of lasers," Proc. IEEE, 5_8,

1342-1357, 1970.

34
[14] H. Statz, "On the conditions for self-locking of modes

in lasers," J. Appl. Phys . , 38^, 4648-4655 , 1967.

[15] A. Scavennec, "Synchronisation des modes du laser continu

Yag:Nd," Unpublished.

[16] A. Scavennec, "Production d' impulsions lumineuses

recurrentes par couplage des modes d'un laser a gaz," _ These Ingenieur- Docteur , Universite de Paris, Centre

d'Orsay, 19 72 .

[17] D. A. Jennings, Private communication.

[18] D.J. Kuizenga, "Mode locking of the cw dye laser," Appl.

Phys. Letters, 19_, 260-263, 1971.

[19] D.J. Kuizenga, A.E. Siegman, "FM and AM mode locking of

the homogeneous laser," IEEE J. Quantum Electron., 6_,

694-708, 1970.

A correction on the pulse width versus modulation index

dependence is reported in:

D.J. Kuizenga, D.W. Phillion, T. Lund, A.E. Siegman,

"Simultaneous Q-switching and mode locking in the cw

Nd:YAG laser," Opt. Commun . , 9, 221-226, 1973.

[20] B.D. Cook, E.A. Hiedmann, "Diffraction of light by ultrasonic waves of various standing wave ratios," J.A.S.A.

,

3^, 945-948, 1961.

[21] D.J. Kuizenga, Private communication.

[22] E.G. Arthurs D.J. Bradley, A.G, Roddie , "Frequency tunable

transform- limited picosecond dye laser pulses," Appl.

Phys. Letters, 19, 480-482, 1971.

[23] F. O'Neill, "Picosecond pulses from a passively mode locked

cw dye laser," Opt. Commun., 4_, 360-363, 1972.

[24] V.N. Lisitsyn, V.P. Chebotaev, "Hysteresis and 'hard'

excitation in a gas laser," JETP Letters, 7^, 1-5 , 1968 .

[25] E. Sakuma, K.E. Evenson, "Characteristics of tungstennickel point contact diodes used as laser harmonic

generator-mixers," To be published.

[26] R.A. Lawton, A. Scavennec, "A photoconductive detector

with response time in the picosecond range," To be

published.

35
IP o

(D -H a p.

•H 5h fH O

+-> 7)

0

•H

I

(=: o

u

CD

= fluorescence lifetime

= intersystem crossing

lifetime

= triplet lifetime

Figure 2-1. Energy level diagram of Rh 6G.

36
37
optical beam

(a) Schematic view o£ a confined cell.

razor

blades

(b) Free flowing dye cell.

Figure 2-3. Dye cells.

38
dump in g

box

filter

corrugated

tubing

dye

cell

centrifugal reservoir

magnetically cooled by

driven pump city water

Figure 2-4. Dye circulating system.

39
active

(a) medium

gain

Cb) cavity

resonances

c2L

laser

(c) TEM ^ ^ ooq

modes

V

E(t)

(d) £ree-rimning

laser

1

(e)

E(t)

mode

locked

laser

A

\

/ i J . J —>-

Figure 3-1. Laser modes and mode locked output

40
41
42
p

•HOu

•HO

to •H>^O

+J

oj rHoI

•H

43

(a) 2 ns/div., 50 mV/div.

(b) 50 ps/div. , 20 mV/div. The secondary pulses come froii

the detector. The sampler has a 20 ps transition time.

Figure 4-5, Active mode locking, detected pulse waveforms.

45
<3

rH ft P CM

P5 +1 •H . ^ 0) KJ

(D >

II

w

u 6 h 6

LO

CO 10

—

T—

CSl 3

t/i

4-)

CD6

CD

5-1

nJ

CDeo

•H

•P

cti I—I

CDOUo

4->

<]

(->

CDa

Clj iH

ft

(/) •H

^3

LO• o o

C/) trt

O +J CD •H tfl t/l 4-> Ci3 rH

03 rH -P ft

CD fi fH O ^ o O • H o !/1 P o

OS

OS

p • CD

CD f-i o

[/)

O •H

iH

D P 11 fH ce O

c:t,p fH

U

•HP

ft o

vOI

CD

fH • H

0}

46
§ DODCI

(a) Five mirror arrangement.

(b) Three mirror arrangement.

Figure 5-1. Passive mode-locking, experimental setup.

47
(a) Five mirror cavity. The signal is very noisy. The pulse

width is estimated to be 80 ps (FWHM) on this photograph.

20 mV/div. , 50 ps/div.

(b) Three mirror cavity. The pulse width is 74 ps (FWHM) here.

5 mV/div., 50 ps/div.

Figure 5-2. Passive mode locking, detected pulse waveforms.

48
Table 6-1. Comparison of the results obtained with the two mode

locking techniques.

Other Laboratories Our Work

^CFWHM) ^av. ^peak

(ps) (mW) (W)

^(FWHM) ^av. ^peak ^o

(ps) (mW) (W) (A)

Active

Mode locking

Acoustooptic [1]

[1]

Electrooptic [18]

55 50 4

(225 MHz)

70

(75 MHz)

<200

(^^400 MHz)

32 10 2.2 5900

(140 MHz)

Pass ive

Mode locking

DODCI

[2]

[23]

1.5 10 50-100

(83 MHz)

4.3 1 1.7

(140 MHz)

<35* 10 >2 5800

(135 MHz)

*Probably detection- limited.

49
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA

SHEET NBSIR 7^-^47

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Active and Passive Mode Locking of Continuously Operating

Rhodamine 6g Dye Lasers

5. Publication Date February 1974

6. Petfoiming Organization Code

7. AUTHOR(S)

Andre' Scavennec and N. S. Nahman

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs.

DEPARTMENT OF COMMERCE

Boulder. Colorado 80302

10. Project/Task/ Work Unit No. 2722169 & 2725169

11. Contract/Grant No.

12. Sponsoring Organization Name and Address National Bureau of Standards

Department of Commerce

Boulder, Colorado 80302

13. Type of Report & Period Covered Summary FY 73

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Report of re search, performed by Dr. Andre* Scavennec as an NBS guest scientist from

Centre National D'Etudes des Telecommunications, Issy-les-MDulineaux, France

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Using confined and unconfined fluid flow dye cells with suitable mode locking methods

continuously operating Rhodamine 6G dye lasers have been built to produce narrow pulses

(less than kO ps ) at about a 1^+0 MHz pulse repetition rate. For active (acoustic)

amplitude modulation mode locking methods optical pulses were obtained having about a

32 ps pulse width (FWHM). For passive mode locking methods optical pulses of ^35 ps

pulse width (FWHM) were obtained.

A three mirror cavity was used for the active mode locking studies while three and five

mirror cavities were used in the passive studies. The three mirror passive mode locked

laser employed a single dye cell containing both the Rhodamine 6g (gain) and DODCI (loss)

solutes in a single solution having glycol as the solvent. The active and passive mode

locked carrier wavelengths were about 5900 K and 58OO respectively.

17. KEY WORDS (Alphabetical order, separated by semicolons)

DODCI; Dye .Laserj Laser; MDde-Lock; Picosecond; RhodaiBine 6g

18. AVAILABILITY STATEMENT

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